Petrogenesis, Geochemistry, and Geological Significance of the Kongco Granitic Porphyry Dykes in the Northern Part of the Central Lhasa Microblock, Tibet

Abstract

The Kongco area of Nima in the northern part of the Lhasa terrane has a suite of alkaline granitic porphyry dykes associated with Early Cretaceous granites and accompanied by Cu/Mo mineralization. LA-ICP-MS ²⁰⁶Pb/²³⁸U zircon geochronology performed on the dykes produced an age of 104.15 ± 0.94 Ma (MSWD = 0.98), indicating the Early Cretaceous emplacement of the dykes. The dykes exhibit high silica (SiO₂ = 76.22~77.90 wt.%), high potassium (K₂O = 4.97~6.21 wt.%), high alkalinity (K₂O + Na₂O = 8.07~8.98 wt.%), low calcium (CaO = 0.24~0.83 wt.%), low magnesium (MgO = 0.06~0.20 wt.%), and moderate aluminum content (Al₂O₃ = 11.93~12.45 wt.%). The Rieterman index (σ) ranges from 1.93 to 2.34. A/NK (molar ratio Al₂O₃/(Na₂O + K₂O)) and A/CNK (molar ratio Al₂O₃/(CaO + Na₂O + K₂O)) values of the dykes range from 1.06 to 1.18 and 0.98 to 1.09, respectively. The dykes are relatively enriched in Rb, Th, U, K, Ta, Ce, Nd, Zr, Hf, Sm, Y, Yb, and Lu, and they show a noticeable relative depletion in Ba, Nb, Sr, P, Eu, and Ti, as well as an average differentiation index (DI) of 96.42. The dykes also exhibit high FeO_T/MgO ratios (3.60~10.41), Ga/Al ratios (2.22 × 10⁻⁴~3.01 × 10⁻⁴), Y/Nb ratios (1.75~2.40), and Rb/Nb ratios (8.36~20.76). Additionally, they have high whole-rock Zr saturation temperatures (884~914 °C), a pronounced Eu negative anomaly (δEu = 0.04~0.23), and a rightward-sloping “V-shaped” rare earth element pattern. These characteristics suggest that the granitic porphyry dykes can be classified as A2-type granites formed in a post-collisional tectonic environment and that they are weakly peraluminous, high-potassium, and Calc-alkaline basaltic rocks. Positive ε_Hf(t) values = 0.43~3.63 and a relatively young Hf crustal model age (T_DM2 = 826~1005 Ma, ⁸⁷Sr/⁸⁶Sr ratios = 0.7043~0.7064, and ε_Nd(t) = −8.60~−2.95 all indicate lower crust and mantle mixing. The lower crust and mantle mixing model is also supported by (²⁰⁶Pb/²⁰⁴Pb)t = 18.627~18.788, (²⁰⁷Pb/²⁰⁴Pb)t = 15.707~15.719, (²⁰⁸Pb/²⁰⁴Pb)t = 39.038~39.110). Together, the Hf, Sr and Pb isotopic ratios indicate that the Kongco granitic porphyry dykes where derived from juvenile crust formed by the addition of mantle material to the lower crust. From this, we infer that the Kongco granitic porphyry dykes are related to a partial melting of the lower crust induced by subduction slab break-off and asthenospheric upwelling during the collision between the Qiangtang and Lhasa terranes and that they experienced significant fractional crystallization dominated by potassium feldspar and amphibole. These dykes are also accompanied by significant copper mineralization (five samples, copper content 0.2%), suggesting a close relationship between the magmatism associated with these dykes and regional metallogenesis, indicating a high potential for mineral exploration.

1. Introduction

The Tibetan Plateau, the youngest plateau in the world, is an ideal location for geodynamic research and serves as a natural laboratory for studying mineral deposits. It consists of a series of nearly east–west-trending blocks or microblocks and suture zones characterized by a serpentinite mélange (Figure 1a) [1,2,3,4,5,6,7,8,9]. From north to south (Figure 1a), these include the Songpan–Ganzi block (SG), Qiangtang block (QT), Lhasa block (LS), and Himalayan block (HM). These blocks and suture zones, influenced by the subduction of the Tethys Ocean and subsequent continental collisions, host extensive magmatic rocks and mineral resources [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33], attracting significant attention from geologists. The LS, also known as the Gangdese block or the Nyainqentanglha block, is bordered to the north by the Bangong–Nujiang Suture Zone (BNSZ) and to the south by the Indus–Yarlung Zangbo Suture Zone (IYSZ) (Figure 1b) [19,28,34,35,36,37,38,39,40,41]. It is further subdivided by the Shiquanhe–Namucuo serpentinite mélange zone (SNSZ) and the Luobadui–Milashan Fault Zone (LMF) into three subunits: the Northern Lhasa Microblock (NL), the Central Lhasa Microblock (CL), and the Southern Lhasa Microblock (SL) (Figure 1b). The BNSZ is a significant suture zone in the northern part of the LS and extends over 1500 km from east to west. It is characterized by compound rocks, mélanges, and segmented ophiolite fragments (Figure 1b) [42]. It represents the remnants of the Tethys Ocean [43], providing a crucial window into the geodynamic history of the Tethys Ocean during the Mesozoic. Some researchers propose that the Bangong–Nujiang oceanic basin opened in the Late Permian, with oceanic crust subduction occurring during the Early to Middle Jurassic [44,45]. However, due to the complex tectonic features and large-scale features of the region, the exact timing of the closure of the Bangong–Nujiang Ocean remains contentious [2,46,47,48,49,50,51].

In recent years, a series of Cretaceous magmatic activities have been successively discovered in the northern part of the LS [53,54,55,56,57,58,59,60,61,62,63]. The study of these magmatic events provides material for exploring the evolutionary history of the BNSZ and its accompanying mineralization events. However, the geodynamic background of this series of magmatic activities remains controversial, and its contribution to regional mineralization processes remains unclear. The controversy is broadly based on two arguments. The first argument was provided by [29], who proposed that the series was related to a northward subduction of the Neo-Tethys Ocean, while Sun et al. (2024) [64] suggested that the series represented a tectonic–magmatic–mineralization event. Other arguments propose that the magmatic activities in the series were products of magmatic processes during the collisional stages between the QT and the LS [3,26,65,66,67,68,69,70,71,72]. In light of these debates, this study investigates the petrogenesis, geochemistry, and metallogenesis of the Kongco granitic porphyry dykes in the Northern Part of the Central Lhasa Microblock in Tibet. The systematic petrological, mineralogical, geochemical and mineralization study of these dykes may provide crucial material for resolving the tectonic–magmatic–mineralization events in the region.

2. Geology

The study area is located in the Kongco area of Nima County of Nagqu, Tibet, at the northern margin of the CL, bordering the SNSZ in the north (Figure 1c). The study area’s geology is complex and is characterized by a predominant NEE-SEE-trending structure, along with NEE-SWW-trending structures formed by post-collisional extensional processes (Figure 1c). The study area comprises three structural units oriented along the northeast–southwest direction: the Northern Lhasa block (NL), the SNSZ, and the CL (Figure 1c). The exposed stratigraphy in the region primarily consists of a Mesozoic stratum, including the Upper Cretaceous Jingzhushan Formation (K₂j) conglomerates, Lower Cretaceous Langshan Formation (K₁l) bioclastic limestone interbedded with sandstones, Duoni Formation (K₁d) sandstones, limestone, and volcanic rocks, Permian Xiala Formation (P₂x) limestone, Carboniferous Yongzhu Formation (C_1-2y) sandstones, shales and limestone, Middle to Upper Devonian Changshehu Formation (D_2-3c) sandstones interbedded with limestone, and Devonian Daerdong Formation (D₁d) limestone (Figure 1c). The Daerdong Formation is characterized by thin-bedded dark gray limestone, bioclastic limestone and marl rich in fossils including brachiopods (Nowakia acuaria), crinoids, corals, bryozoans and trilobites, indicating deposition in a deep-water basin environment (Figure 1c). The Cretaceous magmatic activity in the study area was intense, with exposed intrusive rocks primarily comprising medium-acidic lithologies, including two phases: Early Cretaceous (~105 Ma) and Late Cretaceous (~90 Ma) (Figure 1c). These magmatites intruded into the strata of the Jingzhushan Formation and Xiala Formation in the form of complex plutons and dykes [26,73,74,75]. Additionally, reports indicate medium-acidic magmatic activity around ~90 Ma and ~102 Ma, suggesting crustal melting transitioning from deep to shallow depths, providing significant evidence for an extensional tectonic setting [76,77,78,79].

3. Granitic Porphyry Dykes and Copper Mineralization

In the study area, granitic porphyry dykes, ranging from 1 to 5 m in width, primarily intrude into the Jingzhushan Formation and Xiala Formation limestone with minor intrusion into the conglomerates. They are spatially adjacent to extensive exposures of Early Cretaceous granite, approximately 1–2 km away [52]. These dykes exhibit an irregular radial distribution, extending 500 m to 1000 m. Locally, the intrusion boundaries are distinct, displaying significant magmatic intrusion chill zones (Figure 2b), and are accompanied by weak copper mineralization (Figure 2b,e). The dyke width can reach up to 5 m or wider in some areas, with local occurrences as stocks. Along the contact zones between the dykes and limestone, skarn alteration has developed, primarily characterized by epidotization and silicification (Figure 2b,d). These zones also exhibit varying grades of pyrite and chalcopyrite mineralization, which developed along fracture surfaces. Under the influence of surface water and oxygenation, malachitization is prevalent.

Overall, granitic porphyry dykes exhibit uneven mineral grains (e.g., quartz and feldspar), transitioning from medium to fine grains (Figure 2a–c). The main minerals present are quartz, plagioclase, and potassium feldspar, with minor occurrences of pyrite as accessory minerals. These dykes are predominantly porphyritic, with quartz comprising approximately 60%, K-feldspar 30%, and plagioclase 10% of the phenocrysts. Quartz phenocrysts are colorless, euhedral to subhedral, and some grains are rounded. K-feldspar phenocrysts are typically perthitic, subhedral–tabular, and some exhibit graphic intergrowth with quartz. Some perthitic feldspars contain inclusions of plagioclase. Plagioclase phenocrysts are colorless, subhedral–tabular, exhibiting slight sericitization on the surface, with a calculated anorthite content of approximately 5%, primarily belonging to sodium-rich plagioclase (acidic plagioclase). The matrix is predominantly granophyric cryptocrystalline or microcrystalline, comprising approximately 70% of the rock, with phenocrysts making up the remaining 30%. Spatially, there is significant variation in phenocryst grain size, ranging locally from 5 mm to 10 mm, with rapid changes in grain size and in some areas as small as 1mm, displaying a blocky structure.

Mineralization is notably significant: within and along the irregular contact zones between the granitic porphyry dykes and the surrounding limestone formations, there are varying grades of pyrite and chalcopyrite mineralization. Additionally, a common feature observed in the outer zones of these contact areas is the presence of “iron–manganese crusts”. The chalcopyrite (copper mineralization) and pyrite (iron mineralization) appear as uneven disseminations and veinlets within and along the contact zones between the intrusive veins and the limestone (Figure 2d). Abundant malachite is visible on the surface (Figure 2e). The mineralized alteration is predominantly characterized by epidotization, silicification and sericitization. Epidotization is widespread, occurring in clusters and irregularly distributed chicken-wire patterns along the contact zones between the veins and the surrounding rock. Strong silicification is prominent in highly mineralized areas, typically filling irregularly shaped masses in fractures or veins of varying widths. Both pyrite and chalcopyrite mineralization (malachitization) can be observed within and around the silicified alteration zones. Accompanying the silicification, varying degrees of sericitization are present, manifesting as fine scales of sericite often associated with quartz and pyrite, forming a type of alteration known as beresitization [71]. These observations suggest that the granitic porphyry dykes likely provided the necessary source materials and heat sources for mineralization. The limestone host rock provided conditions and space for the deposition and unloading of minerals. Nearby occurrences of skarn copper deposits such as Qusanggele and Kongco suggest a geological context conducive to mineralization.

4. Samples and Analysis Methods

In this study, the granite porphyry was taken as the research object, and a total of 9 samples for petrology, mineralogy, geochronology and geochemistry were collected in the area with mineralization. All samples were collected from granite porphyry related to mineralization or the contact zone of granite porphyry and mineralization (e.g., Figure 2d,e). The sampling location was near 87°31′15″ E and 31°28′01″ N and the sample numbers ranged from ZQ01~ZQ09. All samples were subjected to observation, identification and major and trace element analysis. One sample (ZQ05) was subjected to zircon U-Pb chronology and Lu-Hf isotope analysis, while three samples were subjected to Rb-Sr, Sm-Nd and Pb isotope testing.

Major and trace element analysis was performed by X-Ray Fluorescence spectroscopy (XRF) and Agilent 7700e ICP-MS, performed at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) at the China University of Geosciences (Wuhan, China), using AGV-2, BHVO-2, BCR-2 and RGM-2 as international standards. Zircon sorting, backscattered electron photography, zircon cathodoluminescence image analysis, zircon U-Pb isotope dating and trace element content were simultaneously analyzed by LA-ICP-MS at the Wuhan Shangpu Analysis Technology laboratory. The GeolasPro laser ablation system was composed of a COMPexPro 102 ArF 193 nm excimer laser and a MicroLas optical system and ICP-MS model Agilent 7500a. The internal standard used in the analysis was the international zircon standard 91500 (1062 ± 4 Ma) [80], and the detailed instrument parameters and analysis procedures can be found in [81]. The laser beam spot for this analysis was 32 μm. The offline processing of the analysis data was performed by the software ICPMSDataCal [82,83]. Zircon in situ micro-Lu-Hf isotope testing was performed using a combination of Neptune-LA-MC-ICP-MS and Geolas Pro laser ablation systems, and the detailed test procedure can be found in Meng et al. (2014) [84]. The diameter of the test beam spot was 32 μm, and all test positions were the same as or close to the U-Pb dating site. The international zircon standard GJ-1 (0.282030 ± 40(2σ); Jackson et al. [85] was used to monitor the Lu-Hf isotope testing. The isotope analysis of Rb-Sr, Sm-Nd, and Pb was completed at the Wuhan Shangpu Analysis Technology laboratory. Isotope analysis was performed using an ISOPROBE-T thermoionization mass spectrometer (Cheshire, UK), single-band, M+, tunable multi-Faraday receiver reception. Mass fractionation was corrected with ⁸⁶Sr/⁸⁸Sr = 0.1194, standard measurement results: NbS987 (0.710250 ± 7), JMC (¹⁴³Nd/¹⁴⁴Nd = 0.512109 ± 3), NBS981, (²⁰⁸Pb/²⁰⁶Pb = 2.164940 ± 15, ²⁰⁷Pb/²⁰⁶Pb = 0.914338 ± 7, ²⁰⁴Pb/²⁰⁶Pb = 0.0591107 ± 2). the whole process background Pb < 100 pg. Laboratory process background: Rb 2 × 10⁻¹⁰ g, Sr 2 × 10⁻¹⁰ g, Sm, Nd less than 50 pg.

5. Results

5.1. Major and Trace Element Analysis

The major and trace element analytical results (Table 1) of the Kongco granitic porphyry dykes showed a high silica content (SiO₂ = 76.22~77.90 wt.%, average 77.09 wt.%), high potassium (K₂O = 4.97~6.21 wt.%, average 5.30 wt.%), high alkali (K₂O + Na₂O = 8.07~8.98 wt.%, average 8.49 wt.%), low calcium (CaO = 0.24~0.83 wt.%, average 0.41 wt.%), low magnesium (MgO = 0.06~0.20 wt.%, average 0.10 wt.%), and a moderate aluminum content (Al₂O₃ = 11.93~12.45 wt.%, average 12.16 wt.%). The Rittmann index (σ) ranged from 1.93 to 2.34, averaging 2.11, indicating a transition from high potassium calc-alkaline to alkaline series granites (Figure 3a,b). The A/NK(molar ratio Al₂O₃/(Na₂O + K₂O)) and A/CNK(molar ratio Al₂O₃/(CaO + Na₂O + K₂O)) ratios ranged from 1.06 to 1.18 and 0.98 to 1.09, respectively, suggesting weak peraluminous characteristics in the granitic porphyry dykes (Figure 3c). This alkaline-to-aluminous character is also observed in other Early Cretaceous A-type granites within the CL (Figure 1b and Figure 3c) [66,75,86,87,88]. The differentiation index (DI) had values ranging from 95.1 to 97.15 with an average of 96.42, indicating a relatively high degree of rock differentiation. The calculated zircon saturation temperatures for the Kongco granitic porphyry dykes ranged from 884 to 914 °C (average 893 °C), suggesting crystallization under relatively high-temperature conditions (Table 1) [89,90,91,92].

Table 1. Major and trace element compositions of the rocks in Kongco.

Sample	ZQ01	ZQ02	ZQ03	ZQ04	ZQ05	ZQ06	ZQ07	ZQ08	ZQ09
SiO2 (wt.%)	76.61	77.45	77.90	77.64	77.44	77.51	76.22	76.31	76.73
TiO2 (wt.%)	0.10	0.06	0.05	0.10	0.10	0.11	0.11	0.12	0.06
Al2O3 (wt.%)	12.23	11.93	11.96	12.21	12.42	12.45	12.19	12.00	12.04
Fe2O3 (wt.%)	0.70	0.35	0.38	0.48	0.26	0.21	0.42	0.62	0.48
FeO (wt.%)	0.19	0.32	0.22	0.08	0.13	0.11	0.16	0.54	0.35
MnO (wt.%)	0.02	0.02	0.01	0.01	0.01	0.01	0.03	0.02	0.02
MgO (wt.%)	0.17	0.06	0.07	0.08	0.09	0.08	0.09	0.20	0.09
CaO (wt.%)	0.48	0.43	0.41	0.28	0.26	0.29	0.83	0.46	0.24
Na2O (wt.%)	2.94	2.77	3.46	3.24	3.09	3.49	2.76	3.41	3.56
K2O (wt.%)	5.13	6.21	4.99	5.09	5.37	4.98	5.94	4.97	4.98
P2O5 (wt.%)	0.06	0.01	0.01	0.02	0.02	0.03	0.02	0.02	0.01
Fe2O3T (wt.%)	0.91	0.71	0.62	0.57	0.40	0.33	0.60	1.22	0.87
FeOT (wt.%)	0.82	0.63	0.56	0.51	0.36	0.30	0.54	1.10	0.78
LOI (wt.%)	1.26	0.35	0.50	0.74	0.77	0.69	1.20	0.60	0.52
Total (wt.%)	99.89	99.96	99.97	99.97	99.96	99.95	99.97	99.28	99.08
A/NK	1.18	1.06	1.08	1.13	1.14	1.12	1.11	1.09	1.07
A/CNK	1.09	0.99	1.01	1.08	1.09	1.07	0.98	1.01	1.03
δ43	1.93	2.34	2.04	2.00	2.07	2.08	2.27	2.10	2.15
DI	95.1	97.04	96.9	96.84	96.92	97.15	95.29	95.49	97.1
Q	38.99	36.27	37.47	38.62	38.29	37.45	35.76	36.3	36.4
Kfs	53.04	58.3	55.65	55.82	56.57	57.03	55.42	55.29	58.25
Pl	4.96	3.86	5.4	3.4	2.95	3.66	7.14	5.89	3.31
La (ppm)	/	/	20.2	18.6	9.9	13.3	31.1	/	/
Ce (ppm)	/	/	47.8	42.7	29.8	27.9	68.8	/	/
Pr (ppm)	/	/	5.51	4.74	2.22	3.05	7.74	/	/
Nd (ppm)	/	/	20.8	16.6	7.9	11.0	25.7	/	/
Sm (ppm)	/	/	5.78	3.81	1.77	2.43	5.99	/	/
Eu (ppm)	/	/	0.076	0.151	0.129	0.182	0.141	/	/
Gd (ppm)	/	/	6.21	4.25	2.00	2.42	5.70	/	/
Tb (ppm)	/	/	1.191	0.810	0.439	0.505	1.026	/	/
Dy (ppm)	/	/	8.40	5.79	3.54	3.77	7.01	/	/
Ho (ppm)	/	/	1.78	1.21	0.79	0.85	1.44	/	/
Er (ppm)	/	/	5.44	3.86	2.63	2.82	4.46	/	/
Tm (ppm)	/	/	0.918	0.669	0.484	0.485	0.715	/	/
Yb (ppm)	/	/	6.13	4.42	3.20	3.30	4.95	/	/
Lu (ppm)	/	/	0.903	0.665	0.483	0.497	0.697	/	/
Y (ppm)	/	/	56.6	39.3	26.3	28.8	45.5	/	/
Rb (ppm)	/	/	270	282	227	249	381	/	/
Ba (ppm)	/	/	31	84	122	120	56	/	/
Th (ppm)	/	/	44.2	39	31.4	32.9	46.3	/	/
U (ppm)	/	/	5.06	5.25	5.24	5.72	5.22	/	/
Nb (ppm)	/	/	32.3	20.1	11.6	12.0	24.4	/	/
Ta (ppm)	/	/	3.75	2.47	1.75	1.81	2.69	/	/
Sr (ppm)	/	/	12.1	24.2	36.8	33.4	14.4	/	/
Zr (ppm)	/	/	102	112	97.1	99.0	146	/	/
Hf (ppm)	/	/	5.19	4.42	3.59	3.56	5.31	/	/
Ga (ppm)	/	/	19.0	16.6	14.6	15.0	18.7	/	/
Sc (ppm)	/	/	2.12	2.53	2.37	2.49	3.04	/	/
Pb (ppm)	/	/	59.2	39.7	36.7	30.6	32.0	/	/
ΣREE	/	/	131	108	65	72	165	/	/
LREE/HREE	/	/	3.24	4.00	3.82	3.95	5.36	/	/
LaN/YbN	/	/	2.24	2.86	2.11	2.73	4.26	/	/
δEu	/	/	0.04	0.11	0.21	0.23	0.07	/	/
δCe	/	/	1.08	1.08	1.48	1.03	1.05	/	/
TZr(°C)	/	/	884	892	887	889	914	/	/

δ43 = (Na₂O + K₂O)²/(SiO₂ − 43), DI = Qz + Or + Ab + Ne + Lc + Kp (CIPW Mass Fraction Ratio); A/NK = Al₂O₃/(CaO + K₂O + Na₂O), A/CNK = Al₂O₃/(CaO + K₂O + Na₂O) (mol Concentration ratio); δEu = 2 × Eu_N/(Sm_N + Gd_N); δCe = 2 × Ce_N/(La_N + Pr_N); Tzr calculation methodology by Watson and Harrison (2005) [93]. T_Zr (°C) = 12,900/(2.95 + 0.85M + ln (496,000/Zrmelt) − 273 [90,92,94]. Note: ppm: 10⁻⁶. “/” indicates that there is no test.

Table 1. Major and trace element compositions of the rocks in Kongco.

Sample	ZQ01	ZQ02	ZQ03	ZQ04	ZQ05	ZQ06	ZQ07	ZQ08	ZQ09
SiO2 (wt.%)	76.61	77.45	77.90	77.64	77.44	77.51	76.22	76.31	76.73
TiO2 (wt.%)	0.10	0.06	0.05	0.10	0.10	0.11	0.11	0.12	0.06
Al2O3 (wt.%)	12.23	11.93	11.96	12.21	12.42	12.45	12.19	12.00	12.04
Fe2O3 (wt.%)	0.70	0.35	0.38	0.48	0.26	0.21	0.42	0.62	0.48
FeO (wt.%)	0.19	0.32	0.22	0.08	0.13	0.11	0.16	0.54	0.35
MnO (wt.%)	0.02	0.02	0.01	0.01	0.01	0.01	0.03	0.02	0.02
MgO (wt.%)	0.17	0.06	0.07	0.08	0.09	0.08	0.09	0.20	0.09
CaO (wt.%)	0.48	0.43	0.41	0.28	0.26	0.29	0.83	0.46	0.24
Na2O (wt.%)	2.94	2.77	3.46	3.24	3.09	3.49	2.76	3.41	3.56
K2O (wt.%)	5.13	6.21	4.99	5.09	5.37	4.98	5.94	4.97	4.98
P2O5 (wt.%)	0.06	0.01	0.01	0.02	0.02	0.03	0.02	0.02	0.01
Fe2O3T (wt.%)	0.91	0.71	0.62	0.57	0.40	0.33	0.60	1.22	0.87
FeOT (wt.%)	0.82	0.63	0.56	0.51	0.36	0.30	0.54	1.10	0.78
LOI (wt.%)	1.26	0.35	0.50	0.74	0.77	0.69	1.20	0.60	0.52
Total (wt.%)	99.89	99.96	99.97	99.97	99.96	99.95	99.97	99.28	99.08
A/NK	1.18	1.06	1.08	1.13	1.14	1.12	1.11	1.09	1.07
A/CNK	1.09	0.99	1.01	1.08	1.09	1.07	0.98	1.01	1.03
δ43	1.93	2.34	2.04	2.00	2.07	2.08	2.27	2.10	2.15
DI	95.1	97.04	96.9	96.84	96.92	97.15	95.29	95.49	97.1
Q	38.99	36.27	37.47	38.62	38.29	37.45	35.76	36.3	36.4
Kfs	53.04	58.3	55.65	55.82	56.57	57.03	55.42	55.29	58.25
Pl	4.96	3.86	5.4	3.4	2.95	3.66	7.14	5.89	3.31
La (ppm)	/	/	20.2	18.6	9.9	13.3	31.1	/	/
Ce (ppm)	/	/	47.8	42.7	29.8	27.9	68.8	/	/
Pr (ppm)	/	/	5.51	4.74	2.22	3.05	7.74	/	/
Nd (ppm)	/	/	20.8	16.6	7.9	11.0	25.7	/	/
Sm (ppm)	/	/	5.78	3.81	1.77	2.43	5.99	/	/
Eu (ppm)	/	/	0.076	0.151	0.129	0.182	0.141	/	/
Gd (ppm)	/	/	6.21	4.25	2.00	2.42	5.70	/	/
Tb (ppm)	/	/	1.191	0.810	0.439	0.505	1.026	/	/
Dy (ppm)	/	/	8.40	5.79	3.54	3.77	7.01	/	/
Ho (ppm)	/	/	1.78	1.21	0.79	0.85	1.44	/	/
Er (ppm)	/	/	5.44	3.86	2.63	2.82	4.46	/	/
Tm (ppm)	/	/	0.918	0.669	0.484	0.485	0.715	/	/
Yb (ppm)	/	/	6.13	4.42	3.20	3.30	4.95	/	/
Lu (ppm)	/	/	0.903	0.665	0.483	0.497	0.697	/	/
Y (ppm)	/	/	56.6	39.3	26.3	28.8	45.5	/	/
Rb (ppm)	/	/	270	282	227	249	381	/	/
Ba (ppm)	/	/	31	84	122	120	56	/	/
Th (ppm)	/	/	44.2	39	31.4	32.9	46.3	/	/
U (ppm)	/	/	5.06	5.25	5.24	5.72	5.22	/	/
Nb (ppm)	/	/	32.3	20.1	11.6	12.0	24.4	/	/
Ta (ppm)	/	/	3.75	2.47	1.75	1.81	2.69	/	/
Sr (ppm)	/	/	12.1	24.2	36.8	33.4	14.4	/	/
Zr (ppm)	/	/	102	112	97.1	99.0	146	/	/
Hf (ppm)	/	/	5.19	4.42	3.59	3.56	5.31	/	/
Ga (ppm)	/	/	19.0	16.6	14.6	15.0	18.7	/	/
Sc (ppm)	/	/	2.12	2.53	2.37	2.49	3.04	/	/
Pb (ppm)	/	/	59.2	39.7	36.7	30.6	32.0	/	/
ΣREE	/	/	131	108	65	72	165	/	/
LREE/HREE	/	/	3.24	4.00	3.82	3.95	5.36	/	/
LaN/YbN	/	/	2.24	2.86	2.11	2.73	4.26	/	/
δEu	/	/	0.04	0.11	0.21	0.23	0.07	/	/
δCe	/	/	1.08	1.08	1.48	1.03	1.05	/	/
TZr(°C)	/	/	884	892	887	889	914	/	/

δ43 = (Na₂O + K₂O)²/(SiO₂ − 43), DI = Qz + Or + Ab + Ne + Lc + Kp (CIPW Mass Fraction Ratio); A/NK = Al₂O₃/(CaO + K₂O + Na₂O), A/CNK = Al₂O₃/(CaO + K₂O + Na₂O) (mol Concentration ratio); δEu = 2 × Eu_N/(Sm_N + Gd_N); δCe = 2 × Ce_N/(La_N + Pr_N); Tzr calculation methodology by Watson and Harrison (2005) [93]. T_Zr (°C) = 12,900/(2.95 + 0.85M + ln (496,000/Zrmelt) − 273 [90,92,94]. Note: ppm: 10⁻⁶. “/” indicates that there is no test.

Figure 3. (a) QAP plots of rocks in Kongco; (b) K₂O vs. SiO₂ plots of rocks in Kongco (basemap after [95,96]); (c) A/NK vs. A/CNK plots of rocks in KongCo (basemap after [97]); (d) chondrite-normalized REE patterns(normalizing values for REE are from Boynton, 1984 [98]); (e) mantle-normalized multi-element diagrams (normalizing values for REE and trace elements are from Sun 1989 [99]). Date according to these articles [3,26,60,73,75,86,87,88,100,101,102,103,104]. The solid and dashed lines represent different ranges determined, showing slight variations between them.

Figure 3. (a) QAP plots of rocks in Kongco; (b) K₂O vs. SiO₂ plots of rocks in Kongco (basemap after [95,96]); (c) A/NK vs. A/CNK plots of rocks in KongCo (basemap after [97]); (d) chondrite-normalized REE patterns(normalizing values for REE are from Boynton, 1984 [98]); (e) mantle-normalized multi-element diagrams (normalizing values for REE and trace elements are from Sun 1989 [99]). Date according to these articles [3,26,60,73,75,86,87,88,100,101,102,103,104]. The solid and dashed lines represent different ranges determined, showing slight variations between them.

The Kongco granitic porphyry dykes exhibit similarities to other A-type granites in the CL. They display elevated total rare earth element content (ΣREE = 65 × 10⁻⁶~165× 10⁻⁶, average 108.52 × 10⁻⁶), moderate ratios of light rare earth elements to heavy rare earth elements (LREE/HREE = 3.24~5.36, average 4.07), and LaN/YbN ratios ranging from 2.11 to 4.26 (averaging 2.84). These values indicate slight enrichment in light rare earth elements and slight depletion in heavy rare earth elements, accompanied by a pronounced negative Eu anomaly (δEu = 0.04~0.23, average 0.13) and a weak positive Ce anomaly (δCe = 1.05~1.48, average 1.14). In the rare earth element chondrite-normalized distribution patterns (Figure 3d), all samples exhibited a consistent trend characterized by a gently sloping “V-shaped” curve, typical of A-type granites [105,106]. From the spider diagram of normalized to primitive mantle values (Figure 3e), the Kongco granitic-porphyry dykes are notably enriched in elements such as Rb, Th, U, K, Ta, Ce, Nd, Zr, Hf, Sm, Y, Yb, and Lu, while they are relatively depleted in Ba, Nb, Sr, P, Eu, and Ti. These geochemical characteristics reflect their origin and evolution within a magmatic setting [107,108].

5.2. Zircon U-Pb Age and Lu-Hf Isotopic Compositions

Zircon grains were extracted from the ZQ05 sample (granitic porphyry) and subsequently subjected to U-Pb chronology, trace element, and Lu-Hf isotope tests. The measurement points were specifically chosen from areas displaying clear rhythmic zoning structures. Detailed analysis locations and results are shown in Figure 4 and Figure 5,

Table 2. LA-ICP-MS zircon U-Pb ages.

Sample	Pb	Th	U	Th/U	Isotope Ratio								Age (Ma)				Concordance
	×10−6	×10−6	×10−6		207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	208Pb/232Th	1σ	207Pb/235U	1σ	206Pb/238U	1σ	%
ZQ05-1	22.25	252.60	387.71	0.65	0.0527	0.0027	0.1182	0.0057	0.0164	0.0002	0.0002	1.6119	113.42	5.16	104.69	2.36	91%
ZQ05-10	30.05	376.02	523.38	0.72	0.0482	0.0020	0.1049	0.0042	0.0158	0.0002	0.0001	1.4755	101.25	3.84	101.21	1.84	99%
ZQ05-11	31.58	386.60	515.62	0.75	0.0485	0.0022	0.1084	0.0048	0.0161	0.0002	0.0001	1.3910	104.53	4.41	103.25	2.01	98%
ZQ05-12	52.58	610.22	857.81	0.71	0.0507	0.0017	0.1124	0.0036	0.0162	0.0002	0.0001	1.4659	108.16	3.29	103.33	1.83	95%
ZQ05-13	50.99	539.06	1140.79	0.47	0.0487	0.0017	0.1124	0.0039	0.0166	0.0002	0.0001	2.2228	108.12	3.57	106.43	2.26	98%
ZQ05-14	48.95	540.05	951.95	0.57	0.0485	0.0021	0.1047	0.0041	0.0157	0.0002	0.0001	1.8378	101.06	3.81	100.67	1.76	99%
ZQ05-16	17.94	216.27	313.64	0.69	0.0534	0.0028	0.1173	0.0061	0.0162	0.0002	0.0002	1.5052	112.64	5.52	103.63	2.62	91%
ZQ05-17	48.93	633.05	629.36	1.01	0.0526	0.0020	0.1159	0.0044	0.0160	0.0002	0.0001	1.0351	111.35	4.05	102.09	1.91	91%
ZQ05-2	28.93	268.25	702.81	0.38	0.0521	0.0019	0.1175	0.0042	0.0164	0.0002	0.0002	2.7536	112.78	3.82	105.17	1.74	93%
ZQ05-21	22.87	235.89	431.89	0.55	0.0495	0.0023	0.1140	0.0054	0.0167	0.0002	0.0002	1.9441	109.61	4.88	106.70	2.13	97%
ZQ05-22	35.14	385.01	594.21	0.65	0.0480	0.0021	0.1088	0.0046	0.0165	0.0001	0.0001	1.6331	104.91	4.21	105.38	1.70	99%
ZQ05-23	17.84	200.74	377.43	0.53	0.0482	0.0026	0.1101	0.0055	0.0169	0.0002	0.0002	1.9355	106.05	5.05	107.83	2.49	98%
ZQ05-3	27.80	299.80	514.01	0.58	0.0517	0.0020	0.1164	0.0044	0.0164	0.0002	0.0001	1.7903	111.78	4.01	105.09	1.99	93%
ZQ05-4	23.75	280.40	403.35	0.70	0.0486	0.0025	0.1100	0.0054	0.0165	0.0002	0.0002	1.5069	105.95	4.91	105.40	2.15	99%
ZQ05-5	22.25	231.69	433.52	0.53	0.0483	0.0025	0.1092	0.0055	0.0165	0.0002	0.0002	1.9420	105.24	5.05	105.47	2.19	99%
ZQ05-6	32.63	390.71	528.91	0.74	0.0507	0.0020	0.1122	0.0041	0.0161	0.0002	0.0001	1.4169	108.01	3.78	103.12	1.85	95%
ZQ05-7	19.86	220.72	337.58	0.65	0.0492	0.0033	0.1077	0.0063	0.0162	0.0002	0.0002	1.6737	103.89	5.78	103.71	2.53	99%
ZQ05-8	54.77	623.64	749.34	0.83	0.0491	0.0020	0.1109	0.0042	0.0164	0.0002	0.0001	1.2927	106.79	3.88	105.18	2.07	98%

,

Table 3. LA-ICP-MS zircon REE compositions (ppm).

Sample	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	Ti	Hf
ZQ05-1	0.032	2.46	0.013	0.13	0.52	0.28	2.30	0.86	11.8	4.76	25.9	6.59	74.3	13.8	147	8.37	5776
ZQ05-10	0.62	14.9	0.17	1.76	2.87	0.27	14.2	5.90	82.7	33.6	170	39.7	422	76.0	1078	3.03	10,455
ZQ05-11	0.0000	15.8	0.10	1.37	2.54	0.39	17.4	6.82	93.9	38.2	191	44.2	465	83.2	1208	1.47	10,008
ZQ05-12	0.12	15.6	0.15	2.79	5.60	0.75	34.1	12.8	164	63.0	291	62.7	635	108	1911	3.65	10,100
ZQ05-13	13.8	57.8	5.48	26.7	10.0	0.69	28.4	9.66	121	47.8	229	51.4	532	93.8	1498	3.47	10,364
ZQ05-14	0.69	18.6	0.27	2.26	3.09	0.25	22.9	8.94	126	52.6	260	60.8	660	115	1635	2.93	11,175
ZQ05-16	0.064	9.09	0.12	1.37	3.54	0.21	22.1	7.90	105	40.1	185	40.7	436	78.2	1189	4.00	11,555
ZQ05-17	0.080	2.23	0.0000	0.17	0.57	0.27	2.08	0.75	11.1	4.35	24.1	6.10	66.3	12.7	136	9.89	5711
ZQ05-2	2.64	19.5	0.87	4.87	3.65	0.58	19.0	6.92	95.4	38.0	187	42.9	462	82.7	1214	6.59	9092
ZQ05-21	0.18	15.7	0.23	1.82	3.42	0.24	21.9	9.27	131	53.1	259	60.3	638	113	1658	4.35	10,850
ZQ05-22	5.52	25.6	1.53	7.94	3.16	0.43	15.2	5.79	75.6	30.8	152	35.0	373	67.1	981	3.89	9991
ZQ05-23	0.89	17.3	0.29	2.34	2.75	0.43	20.5	7.88	103	39.8	191	41.6	425	73.4	1222	5.45	9195
ZQ05-3	11.7	42.2	3.70	16.4	5.11	0.43	14.1	5.34	73.8	31.8	171	43.3	502	94.4	1072	0.000	10,177
ZQ05-4	0.26	15.1	0.11	1.22	2.17	0.48	16.5	6.39	91.4	37.3	189	44.6	484	89.6	1200	1.25	9694
ZQ05-5	0.034	11.0	0.16	2.53	5.10	0.87	28.4	10.1	130	51.0	241	53.7	565	99.3	1589	7.27	9453
ZQ05-6	0.066	12.6	0.053	1.11	3.05	0.28	17.8	7.13	99.8	41.2	205	47.1	506	92.3	1297	6.02	9516
ZQ05-7	2.20	22.4	0.80	5.35	5.08	0.57	31.1	11.5	149	57.7	261	55.4	545	92.0	1701	4.70	8986
ZQ05-8	0.25	8.59	0.13	1.89	3.78	0.33	24.0	8.93	114	43.9	196	42.9	419	72.7	1298	6.37	8529

and

Table 4. LA-ICP-MS zircon Lu-Hf isotope compositions.

Sample	Age (Ma)	176Hf/177Hf	176Lu/177Hf	176Yb/177Hf	εHf(0)	εHf(t)	TDM1	TDM2	fLu/Hf
ZQ05-01	104.7	0.2828	0.0015	0.0373	0.9	3.0	654.3	863.2	−0.96
ZQ05-02	105.2	0.2828	0.0012	0.0275	1.5	3.6	623.4	826.3	−0.96
ZQ05-03	105.1	0.2827	0.0013	0.0315	−1.7	0.4	754.5	1004.6	−0.96
ZQ05-04	105.4	0.2828	0.0012	0.0296	−0.4	1.8	698.8	930.9	−0.96
ZQ05-05	105.5	0.2828	0.0014	0.0340	0.4	2.5	670.1	886.7	−0.96
ZQ05-06	103.1	0.2827	0.0015	0.0373	−1.6	0.6	752.7	997.9	−0.95
ZQ05-07	103.7	0.2828	0.0018	0.0432	−0.4	1.7	710.8	934.3	−0.95
ZQ05-08	105.2	0.2828	0.0015	0.0362	−0.3	1.8	701.8	928.3	−0.96
ZQ05-10	101.2	0.2828	0.0017	0.0441	−0.2	1.9	703.0	924.1	−0.95
ZQ05-11	103.2	0.2827	0.0018	0.0433	−1.0	1.1	733.8	965.8	−0.95
ZQ05-12	103.3	0.2828	0.0017	0.0415	−0.7	1.4	719.6	948.4	−0.95

Notes: ε_Hf(0) = (¹⁷⁶Hf/¹⁷⁷Hf/(¹⁷⁶Hf/¹⁷⁷Hf)_CHUR, 0-1) × 10,000; ε_Hf(t) = (¹⁷⁶Hf/¹⁷⁷Hf − (¹⁷⁶Lu/¹⁷⁷Hf)_S × (e^λt − 1))/((¹⁷⁶Hf/¹⁷⁷Hf)_CHUR, 0 − (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR × (e^λt − 1)) − 1) × 10,000; (¹⁷⁶Hf/¹⁷⁷Hf)_i = ¹⁷⁶Hf/¹⁷⁷Hf − (¹⁷⁶Lu/¹⁷⁷Hf)_S × (e^λt − 1)) − 1; T_DM2 = T_DM1 − (T_DM1 − t) × (f_CC − f_Lu/Hf)/(f_CC − f_DM); f_Lu/Hf = ¹⁷⁶Lu/¹⁷⁷Hf/(¹⁷⁶Lu/¹⁷⁷Hf)_CHUR − 1; (¹⁷⁶Lu/¹⁷⁷Hf)_CHUR = 0.0332, (¹⁷⁶Hf/¹⁷⁷Hf)_CHUR, 0 = 0.282772 [109]; (¹⁷⁶Lu/¹⁷⁷Hf)_DM = 0.0384, (¹⁷⁶Hf/¹⁷⁷Hf)_DM = 0.28325 (Griffin et al., 2000 [110]); f_CC = −0.55, f_DM = 0.16, λ = 1.867 × 10⁻¹¹ year⁻¹ [111].

. These characteristics suggest that the zircons extracted from the Kongco ZQ05 sample are primarily magmatic zircons, characterized by their uniform texture, intact crystal morphology and relatively high Th/U ratios (ranging from 0.38 to 1.01 and averaging 0.65). The zircons exhibit dark CL imaging and dense oscillatory zoning under fluorescence microscopy (Figure 4), further confirming their origin within the crystallization history of the granite.

The ZQ05 sample was tested at 23 analysis points, yielding 18 valid data points for ²⁰⁶Pb/²³⁸U surface ages ranging from 101.21 Ma to 107.83 Ma. The weighted average age was calculated as 104.15 ± 0.94 Ma (MSWD = 0.89) (Figure 5 and Table 2). The zircon ages represent the crystallization age of the Kongco granitic porphyry dykes, which is consistent with the U-Pb zircon ages reported by Liu et al. (2022) [101] for Kongco granites. Therefore, the magmatic activity in the Kongco area likely occurred in the Early Cretaceous period, around 104 Ma.

In a similar manner, Lu-Hf isotope analysis was conducted on some selected dating points from the ZQ05 sample. The ¹⁷⁶Yb/¹⁷⁷Hf ratios for zircons ranged from 0.0275 to 0.0441, with an average of 0.0369 (Table 4). The ¹⁷⁶Lu/¹⁷⁷Hf ratios varied from 0.0012 to 0.0018, averaging 0.0015, while the ¹⁷⁶Hf/¹⁷⁷Hf ratios ranged from 0.282724 to 0.282814, with an average of 0.282763. The calculated ε_Hf(0) values based on corresponding zircon ages primarily ranged from −1.69 to 1.49, averaging −0.31. The ε_Hf(0) values were predominantly positive, ranging from 0.43 to 3.63, with an average of 1.80 (Table 4). The two-stage model ages (T_DM2) ranged from 826.3 Ma to 1004.6 Ma, averaging 928 Ma (Table 4, Figure 6).

5.3. Rb-Sr, Sm-Nd, Pb Isotopes

The Rb-Sr, Sm-Nd, and Pb isotope compositions of the granitic porphyry from Kongco are presented in

Table 5. Rb-Sr, Sm-Nd, and Pb isotope compositions.

Sample	t(Ma)	87Rb/86Sr	87Sr/86Sr	(87Sr/86Sr)t	147Sm/144Nd	143Nd/144Nd	fSm/Nd	εNd(t)	(143Nd/144Nd)t	206Pb/204Pb	(206Pb/204Pb)t	207Pb/204Pb	(207Pb/204Pb)t	208Pb/204Pb	(208Pb/204Pb)t	△β	△γ
ZQ03	104	62.1532	0.8182	0.7055	0.1528	0.512411	−0.22	−3.84	0.5123	18.868	18.777	15.723	15.719	39.370	39.110	25.4	43.8
ZQ04	104	17.7325	0.7474	0.7064	0.1350	0.512155	−0.31	−8.60	0.5121	18.779	18.627	15.714	15.707	39.396	39.098	24.6	43.4
ZQ05	104	76.1255	0.8179	0.7043	0.1362	0.512445	−0.31	−2.95	0.5124	18.947	18.788	15.720	15.712	39.514	39.038	24.9	41.8

Notes: (⁸⁷Sr/⁸⁶Sr)_t = ⁸⁷Sr/⁸⁶Sr − ⁸⁷Rb/⁸⁶Sr (e^λt − 1); λ_Rb = 1.42 × 10⁻¹¹t⁻¹; (¹⁴³Nd/¹⁴⁴Nd)_t = ¹⁴³Nd/¹⁴⁴Nd − ¹⁴⁷Sm/¹⁴⁴Nd (e^λt − 1), λ_Sm = 6.54 × 10⁻¹²t⁻¹; ε_Nd(0) = [¹⁴³Nd/¹⁴⁴Nd/(¹⁴³Nd/¹⁴⁴Nd)_CHUR − 1] × 10000; ε_Nd(t) = {[(¹⁴³Nd/¹⁴⁴Nd)_t/(¹⁴³Nd/¹⁴⁴Nd)_(CHUR)t − 1} × 10000; T_DM2 = 1/λ × In{1+[(¹⁴³Nd/¹⁴⁴Nd)_S − ((¹⁴⁷Sm/¹⁴⁴Nd)_S − (¹⁴⁷Sm/¹⁴⁴Nd)_cc) (e^λt–1) − (¹⁴³Nd/¹⁴⁴Nd)_DM]/[(¹⁴⁷Sm/¹⁴⁴Nd)_cc − (¹⁴⁷Sm/¹⁴⁴Nd)_DM]}; (¹⁴³Nd/¹⁴⁴Nd)_(CHUR)t = (¹⁴³Nd/¹⁴⁴Nd)_CHUR−(¹⁴⁷Sm/¹⁴⁴Nd)_CHUR (e^λt−1); f_Sm/Nd = (¹⁴⁷Sm/¹⁴⁴Nd)_S/(¹⁴⁷Sm/¹⁴⁴Nd)_CHUR − 1; (¹⁴³Nd/¹⁴⁴Nd)_CHUR = 0.512638, (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR = 0, 1967, (¹⁴⁷Sm/¹⁴⁴Nd)_cc = 0.1180, (¹⁴⁷Sm/¹⁴⁴Nd)_DM = 0.2135, (¹⁴³Nd/¹⁴⁴Nd)_DM = 0.51315. △β = (β − βM)/βM × 1000, △γ = (γ − γM) / γM × 1000; β = (²⁰⁶Pb/²⁰⁴Pb)t; γ = (²⁰⁷Pb/²⁰⁴Pb)t; β_M = 15.33; γ_M = 37.47. Calculation formula after DePaolo and Wasserburg (1976) [114].

. The ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios range from 17.7352 to 76.1255 (average 52.0037) and from 0.7474 to 0.8182 (average 0.7945), respectively. The calculated (⁸⁷Sr/⁸⁶Sr)t ranged from 0.7043 to 0.7064 (average 0.7054). The ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios ranged from 0.1350 to 0.1528 (average 0.1413) and from 0.512155 to 0.512445 (average 0.512337), respectively. The calculated (¹⁴³Nd/¹⁴⁴Nd)t and ε_Nd(t) ranged from 0.5121 to 0.5124 (average 0.5123) and from −8.60 to −2.95 (average −5.13), respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios ranged from 18.779 to 18.947 (average 18.865), 15.714 to 15.723 (average 15.719), and 39.370 to 39.514 (average 39.427), respectively. The (²⁰⁶Pb/²⁰⁴Pb)t, (²⁰⁷Pb/²⁰⁴Pb)t, and (²⁰⁸Pb/²⁰⁴Pb)t ranged from 18.627 to 18.788 (average 18.731), 15.707 to 15.719 (average 15.713), and 39.038 to 39.110 (average 39.082), respectively.

6. Discussion

6.1. Types of Rocks

A-type granites are characterized by the presence of alkaline dark-colored minerals such as sodic plagioclase, biotite, and sodic amphibole. Chemically, they typically exhibit low Al₂O₃ (~12.40 wt.%), low CaO (~0.75 wt.%), low MgO (~0.20 wt.%), low A/NK value (~1.08), high Ca/Al ratio, and elevated concentrations of high-field-strength elements (HFSEs) [115]. Mineralogical analysis of the Kongco granitic porphyry dykes indicates that they are rich in potassium feldspar (average 56.15% calculated by CIPW norm), with abundant biotite and occasional hornblende. In terms of major element composition, they show a high potassium content (K₂O = 4.97~6.21 wt.%) and high alkalinity (K₂O + Na₂O = 8.07~8.98 wt.%), along with low aluminum (Al₂O₃ = 11.93~12.45 wt.%), low calcium (CaO = 0.24~0.83 wt.%), and low magnesium (MgO = 0.06~0.20 wt.%) (Table 1). Based on the aluminum index, the Kongco granitic porphyry exhibit weakly peraluminous characteristics (A/NK value: 1.06~1.18, A/CNK value: 0.98~1.09) (Figure 3 and Table 1), which are typical features of A-type granites in terms of mineralogy and major element composition [106,115]. In terms of trace and rare earth elements (Figure 3, Table 1), the Kongco granitic porphyry shows a significant negative Eu anomaly (δEu = 0.04~0.23), with a slight positive Ce anomaly and light rare earth enrichment relative to heavy rare earths (LREE/HREE = 3.24~5.36). In the chondrite-normalized rare earth element distribution diagram (Figure 3d), it displays a characteristic “V-shaped” pattern with a rightward slope. Moreover, the Kongco granitic porphyry is enriched in Rb, Th, U, Pb, Zr and Hf and is relatively depleted in Nb, Ta, Ti, P, Ba, and Sr. These geochemical compositions align with those of A-type granites[48,106,116]. They are notably similar to Early Cretaceous A-type granites reported in the CL (Figure 3), distinguishing them from Late Cretaceous I-type granites in the same area (Figure 3). Criteria such as FeO^T/MgO (3.60~10.41) and Zr (97.1 × 10^−6~146 × 10⁻⁶) further support their classification as A-type granites (Figure 7a,b). The formation temperature of A-type granites is generally higher than that of S-type and I-type granites [117,118]. Calculated using a whole-rock zirconium saturation thermometer [93,119], the formation temperature (Tzr) of the Kongco granitic porphyry ranged from 884 °C to 914 °C, falling within the typical range for A-type granite formation temperatures.

A-type granites typically exhibit characteristics such as a non-orogenic, alkaline and anhydrous nature. However, with further research advancements, scholars have found that A-type granites can form not only in mantle plume or hotspot settings but also in post-orogenic tectonic environments [116]. Based on the petrological and geochemical characteristics of A-type granites, as well as differences in their material sources and tectonic backgrounds, petrologists classify them into two subtypes: A1 and A2 [116]. A1-type granites have trace element compositions similar to Ocean Island Basalts (OIBs) and are considered to form in non-orogenic environments (continental rifts or intraplate settings), while A2-type granites have trace element compositions similar to continental crustal material and Island Arc Basalts (IABs), indicating formation in post-collisional environments [64,127,128,129,130]. There is a transitional type between these two categories without clear boundaries [121,131]. A1-type granites typically exhibit very low Y/Nb and Rb/Nb ratios [105], whereas A2-type granites show the opposite with higher Y/Nb and Rb/Nb ratios [132,133,134]. Similarly to the Late Early Cretaceous (105 Ma) A-type granites reported in the CL, the Kongco granitic porphyry has elevated concentrations of Rb (227 × 10⁻⁶~381 × 10⁻⁶) and Y (26.3 × 10⁻⁶~56.6 × 10⁻⁶) and relatively lower concentrations of Nb (11.6 × 10^−6~32.3 × 10⁻⁶), resulting in average Y/Nb and Rb/Nb values of 2.05 and 15.65, respectively. These values place it within the A2-type granite field on ternary diagrams (Table 1, Figure 7c–f) and in the post-collisional granite field on tectonic discrimination diagrams (Figure 8a,b).

From the above observations, the Kongco granitic porphyry is classified as a weakly peraluminous, high-K, calc-alkaline, potassium basalt series of A2 alkaline feldspar granite.

6.2. Genesis of the Kongco Granitic Porphyry

It is generally believed that A-type granites can form from various magmatic sources, primarily including (a) partial melting of residual F- or Cl-rich lower crustal granulites [106,117,137,138,139]; (b) low-pressure dehydration melting of shallow crustal igneous or granitic gabbroic (depth < 15 km) [140]; (c) partial melting of basaltic rocks [141,142,143]; (d) direct formation via mantle-derived mafic magma separation, crystallization, assimilation [144,145,146]; and (e) interaction between mantle-derived and crustal-derived melts [147]. Experimental petrology shows that melts derived from residual fluorine sources typically have higher MgO than TiO₂ content and exhibit strong peraluminous characteristics [148,149]. Similarly to other Early Cretaceous A-type granites reported in the CL, the Kongco granitic porphyry has elevated TiO₂/MgO ratios (0.59~1.33) and weakly peraluminous characteristics, excluding the possibility of partial melting of residual F- or Cl-rich lower crustal granulites (excluding possibilities “a” above). The Ce/Pb, Ce, Rb/Y and Nb/Y characteristics of Kongco granitic porphyry and other A-type granites reported in the CL show affinity with arc volcanic rocks (Figure 8c,d), and features in the Rb-Sr, Sm-Nd, and Pb isotopes indicate the presence of mantle-derived components (Table 5). Therefore, Kongco granitic porphyry cannot solely originate from pure crustal sources (including shallow crustal and lower crustal), excluding possibilities “b” and “c” above (Figure 8). The high silica rocks derived from mantle-derived mafic magma involve the separation and crystallization of mantle-derived mafic rocks on a large scale [150]. Although Kongco granitic porphyry exhibits a high silica content (SiO₂ = 76.22~77.90 wt.%), there are no reports of large-scale Late Cretaceous mafic magmatic activity in the Kongco region. This implies that the likelihood of Kongco granitic porphyry forming directly via magma separation, crystallization, and assimilation processes is minimal.

The geochemical data presented above strongly suggest that the Kongco granitic porphyry resulted from the interactions between mantle-derived melts and crustal melts. The main lines of evidence are as follows: (a) Kongco granitic porphyry is a weakly peraluminous alkaline feldspar granite. Similar coexistence from alkaline minerals to aluminous minerals also appears in other Early Cretaceous A-type granites in the CL (Figure 1b and Figure 3c, [3,74,86,87,151,152]), as well as in A-type granites in southwestern China and northern Argentina [153]. Its formation may be related to crust–mantle interactions and sediment melting. (b) Kongco granitic porphyry and other Early Cretaceous A-type granites in the CL show a trend of crustal contamination in the Rb/Y-Nb/Y plot (Figure 7). The Lu, Yb, Hf, Rb and Sr isotopes of zircons in the Kongco granitic porphyry exhibit characteristics of crustal origin (Table 4 and Table 5). The melting of crust enriched in high-field-strength element accessory minerals can produce magma with high concentrations of Zr, Ce, Y, and Ga characteristic of Kongco granitic porphyry [147]. Additionally, significant Nb depletion in the trace element spider diagram also indicates crustal components in this granite [154]). (c) In the t-ε_Hf(t) diagram (Figure 5), the Kongco granitic porphyry plots between the Chondritic Uniform Reservoir (CHUR) and the mantle evolution curve. It shares a similar source region with other Early Cretaceous granites in the CL, characterized by positive zircon ε_Hf(t) values (0.43~3.63), relatively young zircon Hf crustal model ages (T_DM2 = 826~1005 Ma), relatively low whole-rock Rb/Y and Nb/Y ratios (Rb/Y = 4.77~8.65, Nb/Y = 0.42~0.57; Table 2), and higher whole-rock Ce/Pb values (0.81~2.15; Figure 7), indicating that the Kongco granitic porphyry originated from juvenile crustal material (Figure 7) with the involvement of mantle material. (d) The (⁸⁷Sr/⁸⁶Sr)i and ε_Nd(t) isotopic characteristics (Table 5) suggest a mixture of mantle and lower crustal components for the Kongco granitic porphyry samples. In terms of Pb isotopes (Table 5), samples primarily fall within the field of mixing between Bangong–Nujiang ophiolites and crust–mantle hybrid lead zones.

It is generally believed that element depletion results from crystallization differentiation processes involving minerals that are enriched in those specific elements [124,155,156]. The Kongco granitic porphyry shows depletion in Ba, Nb, Sr, P, Eu, and Ti, indicating significant crystallization differentiation involving iron–magnesium minerals, titaniferous minerals, and phosphorus-rich minerals in its parental magma. Concurrently, the negative correlations observed between major oxides (FeO^T, Al₂O₃, CaO, MgO, P₂O₅ and TiO₂) and elements like Sr with SiO₂ (Figure 9) suggest crystallization differentiation of feldspar minerals, iron–magnesium minerals (such as amphibole and biotite), phosphorus-bearing minerals (such as apatite), and titanium-bearing minerals (such as ilmenite, titanite, and rutile). This indicates significant crystal fractionation during the formation of this granite. The lines of evidence discussed above suggest that the Kongco granitic porphyry is an Early Cretaceous A-type mass, most likely formed through interaction between mantle-derived mafic magmas and the remelting of juvenile lower crust, and that it underwent significant crystallization differentiation before intrusion.

6.3. Structural Environment and Major Element Analysis

As analyzed earlier, the Kongco granitic porphyry, similar to the Late Early Cretaceous (~105 Ma) A-type granites reported in the CL, belongs to the A2-type granites associated with intracontinental collision. They are most likely formed through interactions between mantle-derived mafic magmas and the remelting of juvenile lower crust during post-collisional processes, where mantle material is thought to have been involved in petrogenesis through mechanisms such as slab tearing and lower crustal delamination [157]. There is controversy regarding the geodynamic context of the widespread Early Cretaceous magmatism in the northern and central parts of the Lhasa terrane [126,151,158,159,160]. (1) It has been attributed to the northward subduction of the Yarlung Tsangpo Neo-Tethyan oceanic crustal slab [17,161]; (2) it has been theorized to have formed due to southward subduction or slab detachment of the Bangong–Nujiang oceanic crustal slab [162,163,164,165,166]; (3) other have claimed that these magmatic activities are products of post-collisional processes between the Qiangtang terrane and the Lhasa terrane [167,168].

Zhang et al. (2004, 2012) [169,170] argue that the northward subduction of the Yarlung Tsangpo Neo-Tethys Ocean involved both low-angle subduction and increasing subduction angles, triggering magmatic eruptions in the central Tibetan during the Cretaceous period. In the low-angle subduction model, early subduction angles were small, resulting in relatively minor magmatic activity [169,170]. However, recent discoveries of extensive Jurassic–Cretaceous magmatism in the southern Lhasa terrane contradict the low-angle subduction model of oceanic crust slabs [72,122,123,126,171,172]. In contrast, Dai et al. (2015) [120] proposed that the northward subduction of the Yarlung Tsangpo Neo-Tethys was a normal deep subduction, and slab-tearing events triggered Early Cretaceous magmatic eruptions. Considering significant crustal shortening in the Lhasa terrane after the Cretaceous, during the late Early Cretaceous, the distance between the northern magmatic arc and the southern Yarlung Tsangpo Neo-Tethys subduction zone exceeded 600 km [44,173,174], which is difficult to explain with a normal subduction model. Therefore, we argue that the formation of the Early Cretaceous Kongco granitic porphyry was not related to the evolution of the Yarlung Tsangpo Neo-Tethys Ocean. Recent studies on the SNSZ indicate that it formed as a back-arc basin during the Bangong–Nujiang oceanic subduction [78,126,175]. Its small scale and rapid evolution suggest that the subduction processes in this basin were unrelated to the large-scale magmatic activity in the northern part of the Lhasa terrane [176].

The exact timing and subduction direction of the closure of the Bangong–Nujiang oceanic basin are still debated [13,19,23,51,52,54,67,177,178,179,180,181,182]. For example, Song et al. (2019) [183] proposed that during the Early Jurassic, the Bangong–Nujiang oceanic basin subducted northward beneath the Qiangtang block, while the collision between the Lhasa and Qiangtang blocks likely occurred around 140–130 Ma through arc–arc “soft” collision. Additionally, records from the Nima area in the northern part of the Lhasa block indicate a transition from marine to non-marine environments between 125 Ma and 118 Ma [174], suggesting a prior “soft” collision between the Lhasa and Qiangtang blocks. Chen et al. (2014) [86] also argued that the extensive magmatic activity around the Shenzha area in the central part of the Lhasa block at approximately 113 Ma resulted from southward subduction of the Bangong-Nujiang oceanic plate rupture. Moreover, evidence has been provided for the early Early Cretaceous (138~134 Ma) indicating that the Bangong–Nujiang ocean had not yet completely closed [184]. Furthermore, reports of extensive distribution of conglomerates from the K₂j Formation imply that since the late Early Cretaceous, the northern margin of the Lhasa block has entered a stage of intracontinental collision [43]. Despite significant discrepancies in these studies, it can be considered that during the late Early Cretaceous, the northern margin of the Lhasa block was in a collisional environment.

The extensive development of Mesozoic mildly acidic magmatism on both sides of the Bangong–Nujiang suture zone in Tibet exhibits a wide range of ε_Hf(t) values during the Cretaceous (Table 4). From the early to the late Early Cretaceous (~105 Ma), ε_Hf(t) abruptly increases from negative to positive values, indicating an increased mantle contribution to the magmas. Subsequently, from the late Early Cretaceous (~105 Ma) to the Late Cretaceous (~65 Ma), ε_Hf(t) decreases to as much as negative values, suggesting a reduction in mantle contributions to the magmas. This implies an event during the late Early Cretaceous where mantle material was introduced into the magma source region, likely associated with post-collisional opening of slab windows. The presence of slab windows beneath the crust generates a suite of distinctive extension-related rock assemblages, including bimodal volcanic rocks, A-type granites, intraplate basalts, and adakites. In the northern part of the Lhasa microblock, collision-related magmatic events associated with extension have been reported during the late Early Cretaceous (~105 Ma), including basalts [79], adakite [185,186], bimodal volcanic rocks, and A-type granites [3,74,86,87,151]. These granitoids exhibit high Ce/Pb ratios, positive ε_Hf(t) values, relatively young zircon Hf crustal model ages, and mantle and lower crustal mixed (⁸⁷Sr/⁸⁶Sr)i, ε_Nd(t) and Pb isotope characteristics, providing evidence of sources from mantle-derived melts and juvenile crustal melts. This is consistent with contemporaneous extension-related magmatism in the region and further supports the existence of slab windows.

Therefore, the late Early Cretaceous magmatism in the northern part of the CL is unlikely to be a product of subduction of the Yarlung–Zangbo Neo-Tethyan oceanic crust slab or ridge subduction, nor is it directly related to the southward subduction and consumption of the Bangong–Nujiang oceanic slab. Instead, it is more likely the result of slab detachment during the collision process between the Bangong–Nujiang oceanic slab and the Qiangtang–Lhasa Block after closure of the subduction of the Bangong–Nujiang oceanic slab. During the late Early Cretaceous, under the collisional process of the Qiangtang-Lhasa Block, detachment of the subducted slab formed slab windows, causing mantle material to rise through the slab window and extension within the overlying lithosphere. Interaction between mantle-derived melts and juvenile crustal melts resulted in the formation of a series of A-type granites in the northern part of the Lhasa microblock within the Qiangtang-Lhasa Block (Figure 7).

6.4. Mineralization Analysis

As shown in Figure 2e, within and around the contact zones and mechanical weak planes of the Kongco granitic porphyry dykes, there are extensive occurrences of botryoidal textured malachite mineralization. Malachitization along microfracture surfaces is prominently developed and with varying grades of malachite patches within the Kongco granitic porphyry dykes. The close spatial relationship between this vein and mineralization suggests that the porphyry dykes had a genetic association with primary copper sulfide mineralization, which was later subjected to oxidation along the fractures in the Kongco granitic porphyry. What is more, within the mineralized alteration zone on both sides of the Kongco granite porphyry, our team collected five mineralized samples, and chemical analysis showed an average Cu content of 0.2% in the five samples (unpublished data). Combined with previous studies, these results indicate that this tectonic magmatic event during the Cretaceous was associated with varying grades of copper mineralization in the western section of the Gangdese belt. Therefore, the Cretaceous porphyry veins in the western section of the Gangdese belt exhibits significant mineralization potential, suggesting considerations for exploration efforts.

7. Conclusions

(1)

The crystallization age of the Kongco granitic porphyry dykes is ~105 Ma, representing a late Early Cretaceous, weakly peraluminous, high-K, calc-alkaline, potassium basalt series of A2 alkaline feldspar granite.

(2)

The Kongco granitic porphyry formed during the collision of the Qiangtang–Lhasa Block following the detachment of the subducted slab, inducing a partial melting of the crust due to asthenospheric upwelling. It subsequently underwent significant potassium feldspar- and biotite-dominated fractional crystallization.

(3)

The close spatial relationship between this vein and mineralization suggests that the porphyry dykes had a genetic association with primary copper sulfide mineralization. This suggests that the porphyritic bodies exhibit significant mineralization potential and promising prospects for exploration.